Balanced audio in hack for HV20 and other unbalanced stereo inputs

[Halsu]

Hi!

I thought i'd share a lttle trick i came up with last week - how to get balanced audio into HV20.

In balanced XLR michrophone cables there's three pins: ground (1), plus two "hot" ones (2,3). The signal one gets from ground and either one of the two hot pins is the same, except the signal phase is opposite. Balanced input is just the difference of these two signals - which one gets by inverting one of them again, then combing them. All the buzzing etc. noise the cable gathers gets phase cancelled, and the signal gets amplified by 6dB. The result is buzz free audio. This is MUCH better than unbalanced, especially if the cables are long, if there's other electrical equipment around etc.

Enough of theory...

One can turn HV20's stereo miniplug michrophone input into a (fake) balanced mono input with a simple cable:

XLR pin 1 > 3.5mm TRS plug sleeve
XLR pin 2 > 3.5mm TRS plug ring
XLR pin 3 > 3.5mm TRS plug tip

A michrophone connected to HV20 with the above custom cable will record a stereo sound, where the signal of L and R is in opposite phase, but all the noise and buzz is in the same phase.

Here's the catch: When the channels are combined, one gets a mono signal with very little usable audio, and a lot of buzzing. But when either the left or the right channel's phase is inverted before combining the channels - TADAA!!

...clean audio without buzz and noise.

[mattias]

yeah, i've tried it and it sort of works. i mean it works but i could never hear much of a difference. i guess it's because the two channels aren't electrically balanced so the noise even though it's the same is quite different on the two channels, or because i didn't have long enough cable runs to get cable induced noise. in theory it definitely works.

/matt

[d1g1t]

Here's the catch: When the channels are combined, one gets a mono signal with very little usable audio, and a lot of buzzing. But when either the left or the right channel's phase is inverted before combining the channels - TADAA!!

...clean audio without buzz and noise.

Interesting.

So this final inversion and combining is done in your audio editing (NLE, etc.) software?

-- Kevin

[Halsu]

So this final inversion and combining is done in your audio editing (NLE, etc.) software?

Yep.

I actually wrote* a phase invert VST plugin that can do the stuff easily:

http://eki.pp.fi/temp/VST/Eki_phase_invert.dll

These settings will work:



* okay, i used Synth Edit, didn't actually code anything ;-)

Here's some other VST:s i've done - freqpan is pretty usable for stereo widening, the others are a bit more obscure tests...

http://eki.pp.fi/temp/VST/


***

I came up with the hack for necessity - we have some dimmable lights / old fluorescent fixtures etc. electronically noisy thingjamagics at our studio. That combined with longish unbal mic cable induced some serious buzzing.

The trick works, not only in theory, but also in practice.

[mattias]

Have you compared it to a "real" transformer balancer? I'm not dissing your post, quite the contrary, just curious what the performance is like.

[Halsu]

Have you compared it to a "real" transformer balancer? I'm not dissing your post, quite the contrary, just curious what the performance is like.

Nope. Ain't gots one that would output a signal that i could record with HV20.

I own a Beachtek box, but i think it doesn't have a balancer, just the bare minimum electronics for phantom power. And in any case, it's out on a long shooting trip right now...

What i could do easily, is to record a test with the same mic/cable directly to computer through my Alesis multimix firewire interface.

This would also be nice way to see how HV20 preamps compare to a "real" mic pre. At the same go, i may test the same signal with a Mackie desk preamp.

I'll do some testing the next time i go to the studio, maybe next monday or so.

[lukeness]

Nope. Ain't gots one that would output a signal that i could record with HV20.

I own a Beachtek box, but i think it doesn't have a balancer, just the bare minimum electronics for phantom power. And in any case, it's out on a long shooting trip right now...

What i could do easily, is to record a test with the same mic/cable directly to computer through my Alesis multimix firewire interface.

This would also be nice way to see how HV20 preamps compare to a "real" mic pre. At the same go, i may test the same signal with a Mackie desk preamp.

I'll do some testing the next time i go to the studio, maybe next monday or so.

Please keep us updated. I'm interested to know the results.

[Halsu]

Please keep us updated. I'm interested to know the results.

I will - not Monday yet ;-)

Meanwhile, here's some audio from our last session:

http://eki.pp.fi/digivideo/HV20Audio...cedtest_01.mp3

The same clip four times over:

1. Just the left channel. This is how the track would have sounded with a "traditional" unbalanced XLR-to-TRS cable. Buzzy.

2. The raw sound recorded with custom "fake balanced" cable. Buzzy, now in stereo.

3. The "fake balanced" stereo sound converted to mono. Buzz only - the usable portion of the sound is pretty much gone. This is the part we will remove.

4. The "fake balanced" stereo sound converted to mono, with one channel's phase inverted. Finally, more or less usable sound!

[Halsu]

>>>>>>>>

[Halsu]

Wow - believe it or not, the above <<<<<<<< post was made by my cat Otto. Smart boy, eh. Now it just needs to learn spelling...

;-)

[rick3000]

Couldn't you just use a low/high transformer XLR to 3.5mm cable like the one from Hosa?

It appears from what I've read that you can do long cable runs with it just fine like with a Beachtek. But it lacks the volume control, line/mic and ground switch. It supposedly keeps everything balanced and buzz free until you get to the last 12" of the cable.

[Halsu]

Couldn't you just use a low/high transformer XLR to 3.5mm cable like the one from Hosa?

Yep, it might do the trick too. Didn't know those were readily available. I haven't done that much stuff with unbalanced 3.5mm inputs really - usually when there's a shoot where we record audio, we're using bigger cameras than HV20 anyway.

I'll need to buy one of those and see how well it works, they're cheap it seems. If they're OK, this trick is just a curiosity really - the savings aren't worth the hassle.

It appears from what I've read that you can do long cable runs with it just fine like with a Beachtek. But it lacks the volume control, line/mic and ground switch. It supposedly keeps everything balanced and buzz free until you get to the last 12" of the cable.

I own the Beachtek DXA-6A box, which has the goodies you mention. But it's borrowed out on a shoot for the next moth or so.

I took a quick look at the specs on their site, and it looks like it IS a transformer balancer after all, i remembered wrong. Which is good news for me too ;-)

****

I guess it might be a good idea to postpone the tests until i get my beachtek box back, as well as get one of those transformer cables - that way i have all bases covered... I guess the examples on my above post will do until then.

[rick3000]

I'm interested to see how your next tests go. Because potentially with an impedance transformer ($16), an adjustable attenuator that does -40db ($20), and a ground hum isolator ($10) you could accomplish the same results as a beachtek for $50. And with a $5 stereo to dual mono adapter you could get two of these for under $100. That's significantly less than buying a beachtek, but it all depends on whether or not it will work.

[mattias]

impressive. you can hear from the "noise track" that the channels aren't completely balanced since then we'd hear nothing of the dialogue, but it's better than i thought. i built my own amp based balancer once and that didn't balance as well even though i used high quality input resistors. i'm still working on that project though since it's technically a preamp so it also lets me bypass the noisy one in the hv.

/matt

[Halsu]

One more thing i'll try is a passive DI (direct injection) box, connected in reverse - mic to balanced output, balanced input to HV20 mic in. In theory, it could work. I have a few of those at the studio.

Those can be very cheap too, they start from around 10 euros (15 bucks).

[Halsu]

Meanwhile, here's some audio from our last session:

http://eki.pp.fi/digivideo/HV20Audio...cedtest_01.mp3

Here's where that bit was for, a small Audio Ball Chair video clip for Designer Eero Aarnio's web site - it's online now, click the New design > play video link:

http://www.designeeroaarnio.com/epag...ObjectID=44268

Here's just the video at a bit better quality:
http://spacewhale.pp.fi/EeroAarnio/B...NLINE_V002.mov

Shot with HV20, on greenscreen, cinemode 25p, using external mic with balanced audio hack cable. The site's intro video is HV20 / greenscreen too.

[PadawanGeek]

Is it still possible to monitor clear audio / audio levels right in the HV20/30 with this configuration?

[Halsu]

Is it still possible to monitor clear audio / audio levels right in the HV20/30 with this configuration?

Levels work just as expected - the sound can also be monitored, but it sounds a bit weird on the phones, like there was a surround stereo effect going on. Also, the buzz is still there when monitoring (just as if using a regular mic cable, so not that bad), it only goes away when combining the channels properly.

One must remember to monitor either in stereo, or listen to just one channel - mono signal is bad (see the sound bite above).

[jwsmite]

I've been a member on here for a while and this is my first post cos I just have to say that you my friend are a freakin' genius!

[bluesgeek]

Couldn't you just use a low/high transformer XLR to 3.5mm cable like the one from Hosa?

It appears from what I've read that you can do long cable runs with it just fine like with a Beachtek. But it lacks the volume control, line/mic and ground switch. It supposedly keeps everything balanced and buzz free until you get to the last 12" of the cable.

Halsu, did you ever do a comparison test using a simple 3.5mm transformer cable? And have you tried a DI both as mentioned above, too?

[PadawanGeek]

Halsu, did you ever do a comparison test using a simple 3.5mm transformer cable? And have you tried a DI both as mentioned above, too?

Comparison between the transformer cable and what?

[bluesgeek]

...and his DIY cable hack. (Sorry)

[SystemZERO]

I made one with a molded 1/8th inch plug, which meant I couldn't tell which wire was tip or sleeve. I used the cable and it seemed to record just fine. I don't know if I wired it backward or what, but I'll do a couple tests and try reversing the phase and see what happens.

[Halsu]

Halsu, did you ever do a comparison test using a simple 3.5mm transformer cable? And have you tried a DI both as mentioned above, too?

Greetings from the time machine - been a while ;-)

Nope, haven't done the tests... been busy with other stuff. I now have the Beachtek box back, so i might very well do some testing in near future. Ain't gots the simple 3.5mm transformer cable though - but i coult try the other stuff.

[bluesgeek]

Thanks for checking in. Happy New Year.